Saturated core transient current limiter

ABSTRACT

A transient current limiting device which includes a saturated core reactor having its windings in circuit with a power supply and a load. The magnetic core is biased into saturation for normal load currents and driven out of saturation by abnormally high transient load currents, such as currents caused by semiconductor loads which have been irradiated by high energy electromagnetic radiation. In one embodiment, the reactor comprises a magnetic core in the form of a wound toroid which exhibits a square loop hysteresis curve and includes a permanent magnet for biasing the core into saturation. A number of configurations are disclosed for placement of the permanent magnet relative to the core to saturate the magnetic element. The invention also permits automatic resetting of the magnetic flux density in the core to saturation when the abnormal load current is removed.

United States Patent [4 1 June 20, 1972 Barnes et al.

[54] SATURATED CORE TRANSIENT CURRENT LIMITER [73] Assignee: The SingerCompany, New York, NY.

[22] Filed: Sept. 18, 1969 21 1 Appl. No.: 859,025

POWER SUPPLY Primary Examiner-Robert K. Schaefer AssistantExaminer-William J. Smith I Att0rney- S.A. Giarratana and S. MichaelBender [57] ABSTRACT A transient current limiting device which includesa saturated core reactor having its windings in circuit with a powersupply and a load. The magnetic core is biased into saturation fornormal load currents and driven out of saturation by abnormally hightransient load currents, such as currents caused by semiconductor loadswhich have been irradiated by high energy electromagnetic radiation. inone embodiment, the reactor comprises a magnetic core in the form of awound toroid which exhibits a square loop hysteresis curve and includesa permanent magnet for biasing the core into saturation. A number ofconfigurations are disclosed for placement of the permanent magnetrelative to the core to saturate the magnetic element. The inventionalso permits automatic resetting of the magnetic flux density in thecore to saturation when the abnormal load current is removed.

5 Claims, 16 Drawing Figures LOAD SHEET 10F 3 FIG. I.

POWER SUPPLY FIG.

PATENTEDJUNZO 1372 H MAGNETOMOTIVE FORCE INVENTORS LELL E BARNES 8 DAVIDMORRIS BY I g l I8 0 PATENTEDJUN20|72 I SHEET m 3 '3 671 l FIG. l6.

AMI-H. h

INVENTORS LELL E. BARNES 8 DAVID MORRIS ATTORNEYS BACKGROUND OF THEINVENTION This invention relates to a transient current limiting device.More particularly, this invention relates to magnetic means in circuitwith a power supply and a load to limit current delivered to the loadunder abnormal conditions. Still more particularly, this inventionrelates to a saturated core reactor having its winding in circuit with apower supply and a load wherein the reactor is biased into saturationfor normal load currents, and is driven out of saturation by abnormallyhigh transient load currents.

It has long been a problem in the electronic art, particularly since theadvent of semiconductors, to limit the current delivered from a powersupply to a load under abnormal conditions. A number of ways have beendeveloped to solve the problem of limiting current under such transientconditions. Perhaps the best known is a fused arrangement which resultsin an open circuit when the current flow exceeds a predetermined level.Still other vacuum tube circuits, semiconductor circuits and magneticamplifier circuits have been developed which will either disconnect thepower supply from the load, or which will convert the power supply to aconstant current source to avoid excessive current to the load.Generally, such approaches have produced complex circuitry and weightycircuit elements which are quite slow to react to the presence of anoverload.

It is also known in the art that semiconductors and semiconductorcircuitry may momentarily exhibit a short circuit to the power supplywhen a semiconductor is bombarded with high H energy radiation, such asgamma rays. Essentially, the shorting effect is caused when theelectromagnetic radiation frees electrons in the semiconductor material.The free electrons cause the semiconductor element to become a shortcircuit load as to the source. Many types of radiation will produce thiseffect, including bombardment with neutrons, electrons, laser beams andgamma rays.

While vacuum tubes and vacuum tube circuits are not generally assusceptible to electromagnetic radiation as semiconductors, vacuum tubesand vacuum tube circuits are not entirely satisfactory in protectingsemiconductor circuitry from transient currents caused by radiationbombardment. In general, such vacuum tube devices are unsuitable becauseof the high power supply required and because of their relatively greatweight which precludes their use in present day circuits.

relatively slow to operate when considered in complex highspeedcircuits, such as computer circuits or specialized communicationcircuits. In particular, it is a problem in this art to provide atransient current limiting device which will preclude delivery ofabnormally high currents to the load in relatively fast times, such asin nanoseconds. For example, in one particular embodiment, it is knownthat the duration of transient currents caused by radiation on the loadexisted on the order of 1-1 0 microseconds so that in a veryparticularized environment, it was a problem to preclude transfer of thetransient current to the load for a very short time.

Accordingly, it is an object of the invention to provide a transientcurrent limiting device.

It is another object of the invention to provide magnetic means incircuit with a power supply in a load to limit the transfer of currenttherebetween under abnormal load conditions.

It is still a further object of this invention to provide a saturatedcore reactor which is biased into saturation for normal load currentsand which is driven out of saturation for abnormally high currents.

It is another object of this invention to provide a transient currentlimiter which comprises a saturable core having a square loop hysteresischaracteristic and which is biased by a permanent magnet.

It is another object of this invention to provide a current Moreover,known solutions to current limiting problem are i loop hysteresischaracteristic which is biased into saturation by a permanent magnet.

. Other and additional objects of the invention will become apparentfrom the perusal of the accompanying drawings and the consideration ofthe detailed specification which follows.

SUMMARY OF THE INVENTION Directed to a solution to the problem oflimiting current between a power supply and load, particularly under theconditions previously discussed, this invention comprises a magneticdevice which is connected in series with a load to limit current in theevent of a transient short circuit. The magnetic device is a core biasedinto saturation for normal load currents and driven out of saturationfor abnormally high load currents. As the flux in the core reverses fromsaturation in one direction to saturation in the other, the high loadcurrent is delayed, and thus limited, particularly due to the domainswitching of the iron core.

In the disclosed embodiments, the winding about the magnetic core is incircuit with the source and the load. A permanent magnet is arranged toinduce a biasing flux in a flux path in the magnetic core. Under normaloperating conditions, the current delivered to the load causes a flux inthe flux path in a direction opposite to that induced by the permanentmagnet, but which maintains the core in its saturated state. Underabnormal conditions, the change in flux in the flux path follows thehysteresis characteristic of the iron core which, in a preferredembodiment, is a square loop characteristic. Thus, the saturatedmagnetic device provides a very low inductance to the circuit undernormal load currents and a time delay for the delivery .of a high loadcurrent under abnormal or short circuit conditions. The device isparticularly useful in preventing high load current caused by shorts dueto electromagnetic radiation because the device itself is not affectedby such radiation. An additional advantage of the invention is that theprotective circuit automatically resets once the abnormal condition isremoved.

BRIEF DESCRIPTION OF THE DRAWINGS In the Drawings:

FIG. 1 is a simplified circuit diagram incorporating thecurrent limitingdevice according to the invention;

FIG. 2 is a perspective view of the reactor core showing the biasingpermanent magnet located in a position coextensive with the geometry ofthe core;

FIG. 3 is a plan view of the core shown in FIG. 2;

FIG. 4 is an end view of the reactor core of FIG. 2 showing the positionof the permanent magnet biasing means;

FIG. 5- shows the square loop B-I-I curve for the toroidal core,illustrating its operation under biasing, normal load, and abnormal loadconditions;

FIG. 6 is a plot of the current supplied by the power supply versus timeover a period including normal operation and abnormal conditions;

FIG. 7 is a plot of flux density in the core versus time underconditions shown in FIG. 5;

FIG. 8 is a reactor core similar to the reactor core shown in FIG. 2wherein the permanent magnet is positioned radially adjacent to thecore;

FIG. 9 is an end view of the reactor core of FIG. 8;

FIG. 10 is a view of the reactor core similar to that shown in FIG. 2wherein the permanent magnet is positioned axially dislimiting devicewhich includes a toroidal core having a square placed from the core;

'FIG. 11 is an end view of the reactor core of FIG. 10;

FIG. 12 illustrates the invention as applied to a 5-1 core arrangementin which the biasing permanent magnet is positioned in the middle leg;

FIG. 13 is a generally C-shaped core arrangement in which the permanentmagnet biasing means is positioned in a portion g of the core;

FIG. 14 depicts the variation of the core of FIG. 12 in which thewindings are positioned on the legs of the E element and the permanentmagnet is positioned adjacent the middle leg;

FIG. 15 is an end view of the structure shown in FIG. 14; and

FIG. 16 is a side view of the structure shown in FIG. 14.

DESCRIPTION OF THE PREFERRED EMBODIMENTS An electrical circuit is shownin FIG. 1 which illustrates one embodiment for incorporating the currentlimiting means according to the invention. A source of power providescurrent in a series circuit by way of leads 17 and 18 to load 11.Current limiter 12, according to the invention, is in series circuitbetween the power supply 10 and load 11. Current limiter 12 isdiagramatically illustrated as including a core 14 of saturable magneticmaterial. Winding 15 on the core 14 is in series circuit with the source10 and load 11. A permanent magnet 16 serves to bias the magneticelement into saturation for normal load currents. The core 14 will bedriven out of saturation by abnormally high load currents flowingthrough the winding 15 such as would be caused, for example, by thepresence of electromagnetic radiation on semiconductor load elements.

The current limiter 12 as shown in greater detail in FIG. 2 comprises areactor 20 including toroid 21 of magnetically permeable material withwinding 24 disposed thereon. In this embodiment, toroidal core 21defines faces 22 and 23 to provide a gap in the toroid.

A permanent magnet 25 is positioned in the gap betweerr faces 22 and 23geometrically coextensive with the toroidal core 21. Air gaps of apredetermined width may be defined. between the permanent magnet 25 andthe core 21 accordingl to the requirements of the magnetic circuit, orthe poles of the permanent magnet 25 may be contiguous with thegenerally planar end faces 22 and 23 of the core 21.

As may be seen in FIGS. 3 and 4, the permanent magnet 25. has a magneticnorth pole 26 and a magnetic south pole 27 and is positioned withrespect to the toroidal core 21 so that; poles 26 and 27 are coextensivewith the planar faces 22 and 23 and with the geometrical configurationof the core.

In FIGS. 2 through 4 the core 21, the winding 24, and the magnet 25correspond to the core 14, the winding 15, and thefl magnet 16 in thecircuit of FIG. 1. As an alternate embodi-' ment, the reactor core maybe biased by the presence of an additional winding about the core havinga predetermined number of turns in a manner resembling winding 24instead of by a permanent magnet.

While the reactor core may be made from iron having con ventionalhysteresis characteristics which exhibit saturation levels of fluxdensity, it is preferred that the core be of a material which exhibits asquare loop hysteresis characteristic as shown by the plot of fluxdensity B versus magnetomotive force H in FIG. 5.

When the power supply 10 is delivering no current to the load 11, themagnetomotive force on the reactor core provided by the permanent magnet16 biases the core well into saturation to the position on thehysteresis loop designated by reference numeral 35 as shown in FIG. 5.The winding 15 is connected between the load 11 and the power supply 10with a polarity so the magnetomotive force opposes that of the permanentmagnet 16. Accordingly, when load current flows through the winding 15,the operating point on the hysteresis curve will move back along thelower portion of the hysteresis loop toward zero magnetomotive forcerepresented by the axis 37. When maximum normal power is delivered tothe load, the resulting current through the winding 15 will move theoperating point on the hysteresis loop to the position designated by thereference numeral 36. The core, however, remains in a saturated state.Variation in load current less than the maximum normal value will causeonly small changes in the total flux in the core because the core willremain in saturation.

If the load should exhibit a momentary short circuit, the increasedtransient current flowing through winding 15 will cause the operatingpoint to move along the lower portion 30 of the hysteresis loop past theknee 38 of the hysteresis loop and into the leg 33 of the hysteresisloop, thus driving the core out of saturation. When the core has beendriven out of saturation into the leg 33 of the hysteresis loop furtherincreases in current will cause large changes in flux in the core. Thus,the winding 15 carrying the load current will exhibit a high value ofinductance opposing further increases in load current. In this manner,the current limiter l2 prevents high transient current from flowing intothe load during momentary shorting of the load 1 l.

The change in flux in the core follows the well known equation:

e KNdrb/dl l where K is a constant N is the number of turns e is theapplied voltage 4) is the flux in the flux path of the reactor core, andt is time From equation I therefore:

=1/KN ed: 2 in which dz represents the change in flux over the timed theintegral.

But qfg BA, where B is the flux density in the flux path of the core andA is the cross sectional area of the core. Therefore, from equation (2):

B l/KNA ed: 3 in which B represents the change in flux density over thetime of the integral.

For a predetermined number of turns and a predetermined cross-sectionalarea of the core, the change in flux density is given by the equation:

For the case in which the applied voltage e is a constant E equal to thepower supply voltage the solution of the integral is:

B K Et K r 5 When a momentary short in the load drives the core out ofsaturation into the leg 33 of the hysteresis loop, the flux density mustchange from -B to +8, amounting to a total change of 28, before the coreis driven all the way to the positive saturation level. Thus, fromequation (5) in order for the total change in flux to equal 28, the timeI must equal 2B,/K Accordingly, if the time of the momentary short isless than 2B,/K the current limiter will prevent the high transientcurrent from being applied to the load. Thus, for short term transientcurrents, the device according to the invention provides an effectivemeans for limiting transient currents for limited times.

FIG. 6, which shows a plot of the supply current from the power supply10 against time, is exemplary of the operation of the circuit when amomentary short occurs. The portion 40 of the curve is the conditionwhen no current is being delivered to the load. At t t, the load isconnected to the power supply and the current rises to a levelrepresented by the portion 41 of the curve. At t 2 in the example ofFIG. 6, the load exhibits a transient short circuit condition.

At a very short time thereafter, I t the start of the flux excursiontime from B, to +8, as shown in FIG. 5 occurs. The time differencebetween and is that necessary for the operating point 36 on thehysteresis loop of FIG. 5 to move from point 36 to the knee 38 of thehysteresis loop at which the core is driven out of saturation and beginsto follow the leg 33 of the hysteresis loop. At time the short circuitends before the core has been driven to positive saturation and thecurrent only increases to the level 42 in response to the momentaryshort circuit. The core is then reset to negative saturation by thepermanent magnet to a range between the points 35 and 36 on thehysteresis loop for normal load currents.

The plot of flux density in the core against time in the exampledescribed with reference to FIG. 6 is shown in FIG. 7. Prior to time I,before the load current is applied, the flux density is the negativesaturation value B,. During the time interval from t t, to t when thecircuit is operating normally, the flux density in the reactor core, asshown by the portion of the curve designated by the reference numeral46, continues at the negative saturation level. From r= t to r t asshown by the portion of the curve designated by reference numeral 47,the flux density remains at the negative saturation level. But duringthe time from r to r t during which the flux is going from negativesaturation toward positive saturation as shown by the portion of thecurve 48, the flux density changes rapidly from B, toward the positivesaturation level +B,. At the end of the short on the load at time theflux density returns, as shown by the portion of the curve 49 in FIG. 7,to its negative saturation level B,. Thus, FIG. 7 illustrates theability of the circuit to reset after the removal of the short circuiton the load.

The speed of resetting is detennined by the leakage reactance for thewinding and the time required for magnetic domain rotation. In atoroidal winding, the leakage reactance may be considered negligible andmay be made quite small by proper winding. Thus, the reset speed becomesnearly a direct function of the magnetic domain rotation from positivesaturation to negative saturation; Such switching may occur in a veryshort time, such as in nanoseconds.

FIG. 8 depicts an alternate embodiment of the invention comprising areactor in the form of a toroidal core 61 having a winding 62 disposedthereon. Core 61 defines an air gap 63 between faces 64 and 65.Permanent magnet biasing means 66 are disposed adjacent to the outercylindrical wall of the core, bridging the air gap 63. FIG. 9 is an endview of the core 61 illustrating the portion of the permanent magnet 66with respect to the core 61.

The embodiment shown in FIGS. 10 and 11 comprises a reactor 70 in theform of a magnetic core 71 having winding 72 disposed thereon. Apermanent magnet 76 is disposed adjacent to a radial wall of the core 71bridging the air gap 73 defined by faces 74 and 75 of the core. Anadditional permanent magnet may be disposed adjacent to the other radialwall of the core 71 axially opposite the pennanent magnet 76.

In the embodiments of FIGS. 8 and 10, the relationship of the variousparameters of the magnetic circuit are such that cores 61 and 71 arebiased into saturation according to the invention as previouslydiscussed.

The embodiment of the invention as shown in FIG. 12 comprises an Ellamination including outer legs 81 and 82 and middle leg 83 having awinding 84 disposed thereon. Permanent magnet 88 is disposed in a gapbetween middle leg 83 and I section 89 to bias the reactor 80 intosaturation as described above. As illustrated, the configuration of thepermanent magnet 88 is such that it is geometrically coextensive withthe face 86 of the middle leg 83. An air gap may be provided between endface 86 and the adjacent pole of permanent magnet 88, as well as betweenthe opposite pole of pennanent magnet 88 and the adjacent wall of the Isection 89.

FIG. 13 is another embodiment of the current limiter comprising aC-shaped core 90 having winding 91 disposed thereon. The C-shaped coredefines a gap in which a permanent magnet 93 is positioned. As in thepreceeding embodiments, the strength of permanent magnet 93 is such tobias the core 90 into saturation under normal operating conditions.

In the embodiment of the invention shown in FIGS. 14 through 16, an E-Ilamination similar to that shown in FIG. 12 comprises a reactor havingouter legs 101 and 102 and middle leg 103. windings 105 and 106 areconnected in a series aiding relationship and are disposed on legs 101and 102 respectively. An I section 107 is disposed to bridge the endfaces of the legs 101, 102 and 103 and a permanent magnet 108 isdisposed bridging an air gap defined between middle leg 103 and 1section 107.

The operation of the current limiter of FIGS. 12 through 15 is inaccordance with the discussion of the FIG. 2 and the curves shown inFIGS. 5 through 7 and accordingly is not repeated in detail here.

Each of the above described current limiters will effectively protectagainst momentary short circuits in the load. The current limiters areparticularly useful in protecting against sh ort circuits caused byintense radiation such as gamma radiation because the current limitersthemselves are virtually unaffected by such radiation.

What is claimed is:

1. An electrical circuit including:

a. a direct current source of power,

b. a load, and

c. magnetic means in circuit with said source and said load for limitingthe current between said source and load in the event of a transientshort circuit, wherein said magnetic means comprises an elongatediscontinuous magnetic core ring member having a longitudinal axis andhaving a center portion and having a pair of axially spaced opposite endportions defining an air gap therebetween, a permanent magnet disposedadjacent to both said end portions of said core and arranged to providean elongate flux path coaxially therewith and having sufiicient strengthfor biasing said magnetic core into saturation for normal load currents,and a winding wound on said core center portion and connected betweensaid source and said load and arranged to provide a second elongate fluxpath coaxially therewith, said second flux path being directed oppositeto said first flux path and having sufficient strength so that said coreis driven out of saturation by abnormally high load currents flowingthrough said winding.

2. The circuit as defined in claim 1 wherein said core is a toroidalelement and said permanent magnet is disposed in said air gap in such amanner that said magnet is geometrically coextensive with said toroidalelement.

3. The circuit as defined in claim 1 wherein said core is a toroidalelement and said permanent magnet is positioned adjacent to the outercylindrical wall of said toroidal element and bridges said air gap.

4. A circuit as defined in claim 1 wherein said core is a toroidalelement and said permanent magnet is positioned adjacent to a radialwall of said toroidal element and bridges said air gap.

5. A circuit as defined in claim 1 wherein said core ring member is aportion of an assembly, said assembly including an I section element anda generally E-shaped magnetic element, said E-shaped element having anouter pair of legs and a middle leg, said winding being disposed aboutat least one of said legs, said E-shaped element and said I sectionelement being spaced to define said air gap therebetween, and whereinsaid permanent magnet is located in said air gap.

1. An electrical circuit including: a. a direct current source of power,b. a load, and c. magnetic means in circuit with said source and saidload for limiting the current between said source and load in the eventof a transient short circuit, wherein said magnetic means comprises anelongate discontinuous magnetic core ring member having a longitudinalaxis and having a center portion and having a pair of axially spacedopposite end portions defining an air gap therebetween, a permanentmagnet disposed adjacent to both said end portions of said core andarranged to provide an elongate flux path coaxially therewith and havingsufficient strength for biasing said magnetic core into saturation fornormal load currents, and a winding wound on said core center portionand connected between said source and said load and arranged to providea second elongate flux path coaxially therewith, said second flux pathbeing directed opposite to said first flux path and having sufficientstrength so that said core is driven out of saturation by abnormallyhigh load currents flowing through said winding.
 2. The circuit asdefined in claim 1 wherein said core is a toroidal element and saidpermanent magnet is disposed in said air gap in such a manner that saidmagnet is geometrically coextensive with said toroidal element.
 3. Thecircuit as defined in claim 1 wherein said core is a toroidal elementand said permanent magnet is positioned adjacent to the outercylindrical wall of said toroidal element and bridges said air gap.
 4. Acircuit as defined in claim 1 wherein said core is a toroidal elementand said permanent magnet is positioned adjaCent to a radial wall ofsaid toroidal element and bridges said air gap.
 5. A circuit as definedin claim 1 wherein said core ring member is a portion of an assembly,said assembly including an I section element and a generally E-shapedmagnetic element, said E-shaped element having an outer pair of legs anda middle leg, said winding being disposed about at least one of saidlegs, said E-shaped element and said I section element being spaced todefine said air gap therebetween, and wherein said permanent magnet islocated in said air gap.